Semi-interpenetrating polymer network scar treatment sheeting, process of manufacture and useful articles thereof

ABSTRACT

Elastomeric sheeting materials are described which are formed from the process of (1) creating a membrane comprising a semi-interpenetrating polymer network (&#34;IPN&#34;) of polytetrafluoroethylene (&#34;PTFE&#34;) and polydimethylsiloxane (&#34;PDMS&#34;) by causing a matrix of PDMS to be formed in situ with a matrix of PTFE; (2) causing a surface of substantially pure PDMS to be formed on at least one side thereof; (3) allowing said PDMS compositions to vulcanize; and (4) converting said structure into useful shapes suitable for application to anatomical areas of the body. The product of the process is suitable for the treatment of dermatologic scars, such as those associated with traumatic or surgical injuries of the skin. The pure PDMS layer provides desired therapeutic effects. The semi-IPN membrane provides improved physical integrity, durability, and elastic behavior in comparison to prior art. The combined structure has increased compliancy, a thinner profile, and improved patient comfort features in comparison to existing products.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the treatment of dermatologic scars,and more particularly concerns scar treatment sheeting and otherarticles, and a method of manufacture thereof.

2. Description of the Prior Art

Silicone chemistry has evolved since the early 1900's into a widevariety of systems used for industrial as well as medical applications.Medical grade silicones are usually based on thermoset dimethyl systems,whereby the molecular formula may be represented as follows: ##STR1##

A crosslinking agent is used to create bonds between the hydrogen atomson the methyl groups of adjacent molecules. These silicone formulationsare often supplied as a two(2) part system, wherein one part contains acatalyst for vulcanization and the other part contains the base resinand crosslinking agent. By varying the amount of crosslinking agent, thecrosslink density may be adjusted to achieve desired bulk or surfacequalities of the vulcanized elastomer. Physical strength and durabilitytend to increase, while softness and coefficient of friction decreasewith higher crosslink densities. Materials with a high crosslink densityare relatively slick and tough, but have poor compliancy or"drapability". Silicones with low crosslink densities give a soft gelwith a more adhesive or "tacky" surface. These materials are moredrapable, although they are inherently weak and tend to fragment easilywith low levels of stress.

A number of U.S. Patents and other publications relate to the field ofthe invention, and are as follows:

U.S. PATENT DOCUMENTS

    ______________________________________                                        4,832,009       5/23/89     Dillon                                            4,945,125       7/31/90     Dillon et al.                                     5,066,683      11/19/91     Dillon et al.                                     5,157,058      10/20/92     Dillon et al.                                     ______________________________________                                    

OTHER PUBLICATIONS

Sperling, Interpenetrating Polymer Networks and Related Materials,Plenum Press, New York, 1981, pp. 1-5.

Dillon, ME, Okunski, WJ, "Silon® Non-Adherent Film Dressings onAutograft and Donor Sites", Wounds, 1992, vol. 4, no. 5:203-207.

Dillon, ME, "Silicone and Poly(tetrafluoroethylene) InterpenetratingPolymer Networks: Brief History, Summary of Recent Developments, andDiscussion of Applications", Interpenetrating Polymer Networks, Klempneret al. ed, ACS Books, New York, N.Y., 1991, pp. 393-404.

Perkins, et. al., "Silicone Gel: A New Treatment for Burn Scars andContractures", Burns, 1982, 9, pp. 201.

Quinn, KJ, "Silicone Gel in Scar Treatment", Burns, 1987, 13, pp. 33-40.

This invention relates to the treatment of dermatologic scars associatedwith traumatic or surgical injuries by using silicone elastomermaterials. In many cases, scar formation may be excessive, resulting inraised, textured or colored surfaces. Scars can not only be disfiguring,but also limit range of motion and functionality. Historically, theapplication of pressure to an affected area of the body has been used tominimize these effects, particularly regarding hypertrophic and keloidalscars. Garments made of an elastic textile are used to achieve suchpressure. This method of treatment eventually became a standard of carein many medical institutions, particularly burn treatment centers.

An Australian research group reported using silicone gel under pressuregarments to evenly apply pressure in anatomic depressions, over areas offlexure, and during ambulation (Perkins et al., 1982). Quinn (1987)later found that the efficacy of silicone for scar modification wasunrelated to pressure, in that the silicone material itself had abeneficial effect on the cosmetic appearance and elasticity of scars.The exact biological mechanism of this effect is not well understood.

In recent years, two(2) general types of silicone sheeting products havegained commercial acceptance in the marketplace for scar modificationapplications. One of the first types, Silastic® (Dow CorningCorporation) consists of a soft polydimethylsiloxane ("PDMS") gelmaterial of approximately 0.125 inch (0.32 cm) in thickness. The lowmodulus of elasticity is beneficial by providing surface tack, thuspromoting skin contact on difficult anatomical areas or during movement.The PDMS composition is inherently weak, however, and endures onlyseveral days in practice before breaking apart because of mechanicalagitation. The above mentioned product uses a reinforcing scrim embeddedinto the body of the material to improve durability. This macroscopicmesh complicates the manufacturing process and may cause skin irritationif exposed during use. Although durability is increased, these productsstill disintegrate from normal wear and tear within a matter of weeks.This is a limiting factor in the cost effectiveness of the product inthat the treatment may last for several months, thus requiring numerousrepurchases. Together, the scrim and thickness of the product compromisedrapability and comfort features.

The second type of commercial product, such as Sil-K® (Degania Silicone,Ltd.), consists of a relatively stiff silicone elastomer ofapproximately 0.03 inches (0.08 cm) in thickness. The increased modulusof elasticity provides for increased physical strength and durability,and the lack of a reinforcing scrim simplifies the manufacturingprocess. The material is relatively non-adherent, so adhesive tape istypically used to maintain the position of the material on the body.Although this material may last for the duration of treatment, it doesnot conform well to anatomical areas and does not provide a high levelof patient comfort. This compromises patient compliancy and may limitthe efficacy of the treatment.

Both types of products are relatively occlusive and impermeable tomoisture vapor, which further detracts from patient comfort. There areother silicone-based scar treatment materials which have beencommercialized, each generally falling into one of the two categoriesabove. Some of the important properties of the above two examples ofcommercial products are listed in Table VII.

Definitions

Moisture Vapor Transmission Rate

The rate at which water vapor permeates through a material calculatedgravimetrically and expressed in units of g/m² /day. The test conditionsare 50% relative humidity, 98° F. (37° C.), with an air flow of 650cubic feet per minute over the specimen.

Tensile Strength

The load required to break a test specimen divided by thecross-sectional area of the specimen.

Modulus of Elasticity

The tensile strength of a material at break divided by the elongation atbreak.

Coefficient of Friction

The force, measured in pounds, required to initiate the slide of a 1inch square (6.45 cm²) by 0.5 inch (1.27 cm) thick piece of high densitypolyethylene over a test specimen on a horizontal surface.

Drapability

The distance the edge of a length of material bends when extended oneinch beyond the surface of a ridged support with a corner radius of lessthan 1/16th inch.

SUMMARY OF THE INVENTION

Interpenetrating polymer networks are defined as a blend of two or morepolymers where each material forms a continuous network, each networkinterpenetrating the other (Sperling, 1981). An IPN is therefore a typeof polymer/polymer composite. A true IPN comprises polymeric ingredientswhich are independently crosslinked. Systems wherein only one componentis crosslinked are called semi-IPNs or pseudo-IPNs, such as an IPN of alinear thermoplastic polymer and a thermoset elastomer. For the purposesof this discussion, the terms IPN, semi-IPN, and pseudo-IPN shall beused interchangeably.

Because of the nature of composite materials, synergistic effects may begained by carefully engineering the morphology and chemistry of thepolymers in an IPN. My previous patents on this subject demonstrateincreased strength may be gained without sacrificing other importantproperties, such as breathability. By using PDMS andpolytetrafluoroethylene ("PTFE"), an IPN with a modulus of elasticityand surface chemistry substantially comprised of PDMS may be producedwhile also possessing the strength and durability of PTFE. The resultantmaterial is semipermeable in that it allows moisture vapor transmissionwhile preventing liquid water break-through. This technology hasprovided skin-like bandages and dressing materials for woundcareapplications such as burn treatment (Dillon et al., 1992, FDA 510[k]approval no. K912032).

I have unexpectedly discovered that an IPN of PDMS and PTFE may be usedas the basis of an elastomeric sheet useful for scar treatmentapplications. The subject of this invention allows the opposingproperties of strength and softness in silicones elastomers to besimultaneously achieved; e.g. durability may be combined withdrapability and surface tack. Furthermore, by causing one side of thestructure to have a greater silicone content than the other, disparatesurface properties may be imparted to each surface. The subject of thepresent invention represents an improvement over prior art in that theproduct:

1. is soft and compliant,

2. is strong and durable,

3. has a thinner profile,

4. has disparate surface properties on each side,

5. is moisture vapor permeable, and

6. has greater comfort features.

The manufacturing process lends itself to large-scale production in thatcontinuous rolls of material may be made in virtually unlimited lengths.This provides for rapid and cost effective conversion into die-cutshapes or self-wound rolls.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in cross section of a sheet constructed in accordancewith this invention;

FIG. 2 is a view in top plan of a face mask constructed in accordancewith this invention;

FIG. 3 is a view in top plan of a sheet which includes holes adapted forapplying treatment to the webbed spaces between fingers or toes;

FIG. 4 shows a sheet which has perforations to facilitate the transferof body heat through the sheet to thereby promote patient comfort; and

FIG. 5 shows a view in section of a "finger cot" for application to thedigits of the hand or other anatomical protrusions.

DETAILED DESCRIPTION OF THE DRAWINGS

Several embodiments of this invention are shown in the followingillustrative examples and are not intended to be limiting. The followingtwo-part silicone compositions were used in each example:

                  TABLE I                                                         ______________________________________                                                      Catalyst/                                                       Type:         Resin Ratio                                                                             Description:                                          ______________________________________                                        Silastic ® Silicone                                                                      1:10     High crosslink density, high                          MDX4-4210               modulus of elasticity, high                           Dow Corning Corporation tensile strength, low                                                         coefficient of friction                               Silastic ® Silicone                                                                     1:1       Low crosslink density, low                            Q7-2218                 modulus of elasticity, low                            Dow Corning Corporation tensile strength, high                                                        coefficient of friction                               ______________________________________                                    

EXAMPLE 1

The following blend of PDMS materials was prepared using trichloroethaneas a processing aid:

                  TABLE II                                                        ______________________________________                                        PDMS Material    Weight (g)                                                                              Percent                                            ______________________________________                                        Q7-2218 Part A   150.4     38.9                                               Q7-2218 Part B   156.4     38.9                                               MDX4-4210 Part B 21.8      5.6                                                Trichloroethane  63.7      16.5                                               ______________________________________                                    

An IPN film of PDMS and PTFE was produced in the form of a continuousroll 12 inches (30 cm) wide according to the methods described in U.S.Pat. No. 4,945,125, the disclosures of which are incorporated herein byreference. Approximately 0.9 g of PTFE (Tetratec Corporation) was usedper linear foot of IPN film produced. The IPN film was measured to havea thickness of 0.001 inches (0.0025 cm). The properties of this materialare given in Table VII.

A second blend of PDMS materials was prepared as follows:

                  TABLE III                                                       ______________________________________                                        PDMS Material    Weight (g)                                                                              Percent                                            ______________________________________                                        Q7-2218 Part A   242.5     25.0                                               Q7-2218 Part B   242.5     25.0                                               MDX4-4210 Part A 440.9     45.5                                               MDX4-4210 Part B 44.1      4.5                                                ______________________________________                                    

A casting process was used to apply a 0.01 inch (0.025 cm) surfacecoating of the above PDMS composition to one side of the IPN film. Thecasting process involved passing the substrate through a reservoir ofliquid PDMS prepolymer and using a doctoring roll to meter off acontrolled amount of liquid as to leave a precise thickness of PDMS onthe surface of the IPN substrate. Numerous other methods of achievingsuch a coating will be apparent to those skilled in the art. The coatedmaterial was passed through a tunnel style oven as to initiate thecuring process, then post cured in a closed oven for 3 hours at 158° F.(70° C.) to achieve full vulcanization. FIG. 1 shows a sectional view ofthe construction of this example, with IPN layer 11 and PDMS coating 12.For comparative purposes, the same process was used to create 0.01 inch(0.025 cm) sheet of the same PDMS material without the bottom layer ofIPN film. A paper release liner (H. P. Smith Company) was used as atemporary substrate from which the PDMS sheet was later removed. Some ofthe important physical characteristics of each material produced by thisexample are given in Table VII.

EXAMPLE 2

The following blend of PDMS materials was produced:

                  TABLE V                                                         ______________________________________                                        PDMS Material    Weight (g)                                                                              Percent                                            ______________________________________                                        Q7-2218 Part A   44.0      4.0                                                Q7-2218 Part B   44.0      4.0                                                MDX4-4210 Part A 320.0     29.1                                               MDX4-4210 Part B 32.0      2.9                                                Trichloroethane  660.0     60.0                                               ______________________________________                                    

An IPN film of PDMS and PTFE was produced in the form of a continuousroll 12 inches (30 cm) wide using the PDMS blend of Table V according tothe methods previously described. Approximately 1.3 g of PTFE (TetratecCorporation) was used per linear foot of IPN film produced. The finalIPN was measured to have a thickness of 0.002 inches (0.05 cm).

A second blend of PDMS materials were prepared as follows:

                  TABLE VI                                                        ______________________________________                                                         Weight (g)                                                   PDMS Material    (g)       Percent                                            ______________________________________                                        Q7-2218 Part A   232.5     25.0                                               Q7-2218 Part B   232.5     25.0                                               MDX4-4210 Part A 422.7     45.5                                               MDX4-4210 Part B 42.3      4.5                                                ______________________________________                                    

The process outlined above was used to apply a a 0.036 inch (0.091 cm)surface coating of the above PDMS blend to one side of the IPN film. Aswith the previous example, the material was passed through tunnel styleoven as to initiate the curing process, then post-cured in a closed ovenfor 3 hours at 158° F. (70° C.). Again the same process was used tocreate 0.03 inch (0.075 cm) sheet of the same PDMS material without thebottom layer of IPN film. Some of the important physical characteristicsof the materials produced by this example are given in Table VII.

The above samples are preferred embodiments of this invention. Otherformulations and constructions will be apparent to those skilled in theart. For example, there are many other silicone compositions that wouldbe suitable for the subject of this invention, either in combination orin blends. Dow Corning Silastic® grades MDX4-4515, Q7-2213, Q7-2167,Q7-2168, Q7-2174, Q7-2245, Q7-4840, and Q7-4850 are suitablesubstitutes. Furthermore, various thicknesses of IPN films and siliconecoatings may be useful. The range of suitable IPN thickness is between0.0005 inch (12.5 microns) and 0.02 inch (0.05 cm), and that for thePDMS is between 0.005 inch (0.0125 cm) and 0.25 inch (0.64 cm). Animportant consideration is that the final construction is not so thin asto have a tendency to wrinkle or otherwise be difficult to handle. Thesteps of creating a layered article may be reversed or even consolidatedinto one process.

In addition to various compositions, there are numerous shapes and sizesuseful for the subject of this invention. For example, the invention maybe converted into sheets or rolls for final use. Other embodimentsinclude face-mask designs as shown in top plan view in FIG. 2 whichshows a face mask 21 with cut openings 22 which accommodate the featuresof the face. An additional embodiment is shown in top plan view in FIG.3, wherein a sheet 31 includes holes 32 cut as to apply treatment to theweb-spaces between fingers and toes. Yet another embodiment is shown inFIG. 4 where a sheet 41 has perforations 42 to facilitate the transferof body heat through the sheet 41, thereby promoting patient comfort.

A particularly novel configuration of this invention is a "finger cot"for application to the digits of the hand or other anatomicalprotrusions. FIG. 5 shows a sectional view of this embodiment and showsa mandrill 51 covered by an inventive article 52. This configuration maybe achieved by: (1) conforming--or forming--a PTFE/PDMS IPN articlearound the end of a suitable mandrill, (2) dipping the mandrill in areservoir of uncured polysiloxane, (3) removing the mandrill from thereservoir, (4) allowing the residual surface coating of polysiloxane tovulcanize, (5) removing the polysiloxane coated IPN article by peelingit off of the mandrill as it is turned in-side-out, and (6) (optionally)rolling the mouth of the shaped article outward and back to facilitateapplication by rolling the mouth of the product back down over theanatomical protrusion.

                                      TABLE VII                                   __________________________________________________________________________                     Moisture Vapor                                                                         Tensile                                                                             Modulus       Coefficient of Friction                    Thickness                                                                           Permeability                                                                           Strength.sup.(1)                                                                    of Elasticity                                                                        Drapability                                                                          (Side A/Side B).sup.(2)         Specimen   Inch  g/m.sup.2 /24 hours                                                                    Lbs/in.sup.2                                                                        Lbs/in.sup.2                                                                         Inch   Pounds                          __________________________________________________________________________    Silastic   0.125 19.8     18.4  7.3    0.5    11.3/2.9                        (Dow Corning)                                                                 Sil-K      0.032 41.3     981.4 101.4  0.34   0.5/0.5                         (Degania Silicone)                                                            Example 1  0.001 740.7    2638.5                                                                              1499.1 0.94   1.8/1.8                         IPN Film                                                                      Example 1  0.011 204.2    115.0 38.1   0.94   5.2/3.4                         IPN w/PDMS Coating                                                            Example 1  0.010 331.9    4.0   3.3    0.94   9.0/9.0                         PDMS Sheet Only                                                               Example 2  0.002 770.0    2652.6                                                                              1020.2 0.94   1.1/1.1                         IPN Film                                                                      Example 2  0.038 51.9     55.5  18.8   0.88   13.9/3.6                        IPN w/PDMS Coating                                                            Example 2  0.030 99.3     6.3   4.8    0.94   9.0/9.0                         PDMS Sheet Only                                                               __________________________________________________________________________     .sup.(1) Tensile Tests were performed at a strain rate of 3.34 ×        10.sup.-2 in/sec. Note: the ultimate load required to break a one inch        wide length of the specimens equals the tensile strength multiplied by th     thickness.                                                                    .sup.(2) Side A is applied to the skin.                                  

I claim:
 1. A composite article for treating dermatologic scars,comprisinga first layer of a crosslinked elastomer, and a second layerof a membranous film, wherein the crosslinked elastomer ispolydimethylsiloxane, and wherein the membranous film is asemi-interpenetrating polymer network of polydimethylsiloxane andpolytetrafluoroethylene, the article having a minimum moisture vaportransmission rate of 51.9 g/m² /day, and the article having a minimumdrapability of 0.88 inches.
 2. The article of claim 1,wherein each layerhas surface characteristics that are disparate from the other layer. 3.The article of claim 1,with a modulus of elasticity of less than orequal to 38.1 lbs/in².
 4. The article of claim 1,wherein the layer ofpolydimethylsiloxane is 0.01 inches thick.
 5. The article of claim1,wherein the layer of polydimethylsiloxane is 0.03 inches thick.
 6. Thearticle of claim 1,wherein the semi-interpenetrating polymer network is0.001 inches thick.
 7. The article of claim 1,wherein thesemi-interpenetrating polymer network is 0.002 inches thick.
 8. Aprocess which comprises the steps of(1) forming a membranous film, (2)causing a surface of polydimethylsiloxane to be formed at least one sidethereof to form a composite structure, and (3) converting said structureinto shapes for covering anatomical areas of the body, wherein themembranous film is a semi-interpenetrating polymer network ofpolydimethylsiloxane and polytetrafluoroethylene, the compositestructure having a minimum moisture vapor transmission rate of 51.9 g/m²/day, and the composite structure having a minimum drapability of 0.88inches.
 9. The process of claim 8, including applying said shapes todermatologic scars.
 10. The process of claim 8,wherein the shapes haveholes to accommodate fingers or toes.
 11. The process of claim 8,whereinthe shapes are configured in a face-mask fashion.
 12. The process ofclaim 8,wherein the shapes have perforations to promote heat transferfrom the body.
 13. The process of claim 8,wherein the shapes areconfigured in the fashion of a finger cot.
 14. A composite article fortreating dermatologic scars comprisinga first layer of a crosslinkedelastomer, and a second layer of a membranous film, the membranous filmbeing a semi-interpenetrating polymer network of polydimethylsiloxaneand polytetrafluoroethylene, the crosslinked elastomer beingpolydimethylsiloxane, wherein each layer has surface characteristicsthat are disparate from the other layer, the layer ofpolydimethylsiloxane being 0.01 inches thick, and thesemi-interpenetrating polymer network being 0.001 inches thick.
 15. Aprocess which comprises the steps of(1) forming a layer of a membranousfilm, (2) causing a surface of polydimethylsiloxane to be formed on atleast one side thereof as a layer to form a composite structure, and (3)converting said structure into shapes for covering anatomical areas ofthe body, wherein the membranous film is a semi-interpenetrating polymernetwork of polydimethylsiloxane and polytetrafluoroethylene, whereineach layer has surface characteristics that are disparate from the otherlayer, the layer of polydimethylsiloxane being 0.01 inches thick, andthe semi-interpenetrating polymer network being 0.001 inches thick. 16.The process of claim 15,wherein the shapes have perforations so as topromote heat transfer from the body.
 17. The process of claim 10,whereinthe shapes have perforations to promote heat transfer from the body. 18.The process of claim 11,wherein the shapes have perforations to promoteheat transfer from the body.
 19. The process of claim 13,wherein theshapes have perforations to promote heat transfer from the body.
 20. Theprocess of claim 15,wherein the shapes have holes to accommodate fingersor toes.
 21. The process of claim 15,wherein the shapes are configuredin a face-mask fashion.
 22. The process of claim 15,wherein the shapesare configured in the fashion of a finger cot.
 23. The process of claim16,wherein the shapes have holes to accommodate fingers or toes.
 24. Theprocess of claim 16,wherein the shapes are configured in a face-maskfashion.
 25. The process of claim 16,wherein the shapes are configuredin the fashion of a finger cot.
 26. The process of claim 9,wherein theshapes have perforations to promote heat transfer from the body.
 27. Theprocess of claim 15, including the step of applying said shapes todermological scars.
 28. The process of claim 16, including the step ofapplying said shapes to dermological scars.